Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B9/00—Measuring instruments characterised by the use of optical techniques
  • G01B9/02—Interferometers
  • G01B9/02055—Reduction or prevention of errors; Testing; Calibration
  • G01B9/02062—Active error reduction, i.e. varying with time
  • G01B9/02063—Active error reduction, i.e. varying with time by particular alignment of focus position, e.g. dynamic focussing in optical coherence tomography
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B3/00—Apparatus for testing the eyes; Instruments for examining the eyes
  • A61B3/10—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions
  • A61B3/12—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions for looking at the eye fundus, e.g. ophthalmoscopes
  • A61B3/1225—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions for looking at the eye fundus, e.g. ophthalmoscopes using coherent radiation
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B9/00—Measuring instruments characterised by the use of optical techniques
  • G01B9/02—Interferometers
  • G01B9/02001—Interferometers characterised by controlling or generating intrinsic radiation properties
  • G01B9/02012—Interferometers characterised by controlling or generating intrinsic radiation properties using temporal intensity variation
  • G01B9/02014—Interferometers characterised by controlling or generating intrinsic radiation properties using temporal intensity variation by using pulsed light
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B9/00—Measuring instruments characterised by the use of optical techniques
  • G01B9/02—Interferometers
  • G01B9/02055—Reduction or prevention of errors; Testing; Calibration
  • G01B9/02062—Active error reduction, i.e. varying with time
  • G01B9/02067—Active error reduction, i.e. varying with time by electronic control systems, i.e. using feedback acting on optics or light
  • G01B9/02068—Auto-alignment of optical elements
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B9/00—Measuring instruments characterised by the use of optical techniques
  • G01B9/02—Interferometers
  • G01B9/02055—Reduction or prevention of errors; Testing; Calibration
  • G01B9/02062—Active error reduction, i.e. varying with time
  • G01B9/02067—Active error reduction, i.e. varying with time by electronic control systems, i.e. using feedback acting on optics or light
  • G01B9/02069—Synchronization of light source or manipulator and detector
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B9/00—Measuring instruments characterised by the use of optical techniques
  • G01B9/02—Interferometers
  • G01B9/02055—Reduction or prevention of errors; Testing; Calibration
  • G01B9/02075—Reduction or prevention of errors; Testing; Calibration of particular errors
  • G01B9/02076—Caused by motion
  • G01B9/02077—Caused by motion of the object
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B9/00—Measuring instruments characterised by the use of optical techniques
  • G01B9/02—Interferometers
  • G01B9/02055—Reduction or prevention of errors; Testing; Calibration
  • G01B9/02075—Reduction or prevention of errors; Testing; Calibration of particular errors
  • G01B9/02078—Caused by ambiguity
  • G01B9/02079—Quadrature detection, i.e. detecting relatively phase-shifted signals
  • G01B9/02081—Quadrature detection, i.e. detecting relatively phase-shifted signals simultaneous quadrature detection, e.g. by spatial phase shifting
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B9/00—Measuring instruments characterised by the use of optical techniques
  • G01B9/02—Interferometers
  • G01B9/0209—Low-coherence interferometers
  • G01B9/02091—Tomographic interferometers, e.g. based on optical coherence
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B2290/00—Aspects of interferometers not specifically covered by any group under G01B9/02
  • G01B2290/70—Using polarization in the interferometer

Definitions

• the present invention relates to a device for measuring the contrast of the fringes in a Michelson interferometer operating in the field. It also relates to a method implemented in this device, as well as an eye examination system including such a device.
• Michelson's full-field interferometry is an interferometric technique derived from the classical Michelson montage, which among other things makes it possible to deduce the surface and reflectance profiles of a sample from the interferences between the light reflected by the sample and that reflected by from a reference arm.
• this technique When used with a temporally weakly coherent light source, this technique has a tomographic capacitance in transparent and weakly diffusing media. It is then a coherent optical tomography (OCT) field.
• FIG. 1 thus illustrates the structure of an OCT tomography system by full field interferometry representative of the prior art.
• the full-field OCT has often been used in polarized light.
• the two arms of the Michelson interferometer operate with perpendicular polarizations.
• the interference is only performed on the output arm after projection on a common axis by the use of a 45 degree analyzer.
• the interferometric modulation hitherto necessary for the detection of low amplitude interference, is performed either by moving the moving mirror of the reference arm or by modulating the optical path of one of the polarizations on the output arm of the beam.
• interferometer for example, by the use of a photoelastic modulator prior to the 45-degree analysis.
• Interferogram amplitude measurements under poor signal-to-noise conditions often lead to the consideration of techniques for modulating the interferometric operating difference coupled with synchronous detection techniques.
• the phase state of the interferograms obtained will be temporally modified so as to eliminate an optionally variable DC component of the measured signal, which results from the incoherent combination of the output signals, either due to parasitic reflections or to reflections with gait differences greater than the coherence length of the radiation used.
• the mechanical stability, therefore interferometric, of the entire system imposes high modulation frequencies, it happens that the detection chain, usually a CCD sensor in the case of the full field, can not operate properly.
• a polarizer at 45 degrees of the incident polarization directions, on whose direction the fields coming from the two arms of the interferometer are projected, is usually used. Then appear interference. During this projection, half of the incoming energy is lost, which contributes to degrade the performance of the interferometer.
• the object of the present invention is to remedy this drawback by proposing a device for measuring the contrast of the fringes in a Michelson interferometer, allowing simultaneous obtaining of interferograms in different phase states, which gives access to a measurement of their amplitude and even their phase despite possible instabilities. This invention applies to the case of full field interferograms in polarized light.
• a contrast measuring device comprising means for deflecting two perpendicular polarizations entering in two different emergent directions, these deflection means being disposed within the interferometer in substitution of the single polarizer.
• the lost light is the projection of the fields in a direction perpendicular to the direction of the polarizer. But in this direction, we could also observe interference.
• Replacing the single polarizer with a Wollaston prism allows both outputs to be used as the outputs of two perpendicular polarizers that would be used simultaneously. The entire incoming light is thus used.
• the deflection means may advantageously be in the form of a Wollaston prism, the axes of which are oriented at 45 degrees to the polarizations of the arms.
• This prism is a common commercial device that is usually used to angularly separate linearly polarized radiations.
• the separation angle is a characteristic of the Wollaston prism and can be chosen in a wide range. Mounting a Wollaston prism at the output of a full-field Michelson interferometer thus makes it possible to perform several simultaneous interferograms at different phase states. It makes the extraction of amplitude and phase information of the radiation coming from the interferometer particularly simple and robust.
• the device according to the invention is particularly applicable to the case of full field OCT tomography on mechanically unstable samples on an interferometric scale.
• the extraction of the interferometric contrast of the data is done, given the very low value of this contrast, by the filling of modulation techniques associated with methods of synchronous detection.
• These measures are, by the very principle of modulation, obtained on different dates. To guarantee the coherence of these measurements imposes to be able to guarantee the invariance of the difference of course (except the sought modulation) constant.
• a second objective of the present invention is to solve this problem relating to the variability of the difference in operation. This objective is achieved with the use of a Wollaston prism which allows to obtain at least two measures strictly simultaneous and in opposition of phase.
• Ia Il + I2 + 2.V (II.I2) .cos ( ⁇ )
• Ia Il + I2 + 2.V (H.I2) .cos ( ⁇ + ⁇ / 2)
• Ia Il + I2 + 2.V (II.I2) .cos ( ⁇ + ⁇ )
• Ia It + I2 + 2.V (H.I2) .cos ( ⁇ + 3 ⁇ / 2)
• the interferograms must be consistent, ie the phase shift imposes (0, ⁇ / 2, ⁇ , 3 ⁇ / 2) must be constant in the field.
• the Wollaston prism is used in a pupil plane.
• the half-wave plate acts as an interface between the upstream assembly of the Wollaston prism and the downstream assembly.
• the desired modulation frequencies are generally quite high, which imposes relatively short exposure and reading times. This problem of image rate and exposure time is solved with a simultaneity of measurements which then eliminates any need for speed for the detector.
• the shortness of the pose, if any, can be ensured by the brevity of the illumination, for example a flash, without either the integration of the detector or its reading, are brief or fast.
• an in vivo tomography eye examination system comprising: a Michelson interferometer, performing a full-field OCT assembly, adaptive optics means, arranged between the interferometer and an eye to be examined, performing correction of the wave fronts coming from the eye but also towards the eye, and
• This examination system may further advantageously comprise a sighting device comprising at least one moving target having a shape and a programmable trajectory, this at least one target being displayed on an appropriate screen, visible from both eyes, during the duration of the examination.
• FIG. 1 is a block diagram of a full-field OCT tomography system representative of the prior art
• FIG. 2 illustrates the principle of a Wollaston prism implemented in an improvement device according to the invention
• FIG. 3 schematically illustrates a first two-phase measurement configuration of a contrast enhancement device according to the invention
• FIG. 4 schematically illustrates a second four-phase measurement configuration of a device. contrast enhancement apparatus according to the invention
• FIG. 5 schematically illustrates a practical example of embodiment of an in vivo tomography system incorporating a contrast enhancement device according to the invention.
• FIG. 6 is a diagram of another exemplary embodiment of an in vivo tomography system according to the invention.
• 2 and Ih
• the amount was 2 should be seen as the superposition of a range of radiation 20 that does not interfere with have a fault a sufficiently small difference in path, and a radiation of amplitude at 2 * reflected with a difference in operation less than the coherence length of the source used, which interferes with it.
• Iv l / 2 (a ⁇ 2 + a 2 2 + 2a ⁇ .a 2 * .cos ( ⁇ ))
• Ih l / 2 (a ⁇ 2 + a 2 2 + 2a ⁇ .a 2 * .cos ( ⁇ + ⁇ ))
• Im 2a ⁇ .a 2 * .cos ( ⁇ ) It is this same quantity which one obtains by synchronous detection, but then at the price a temporal dichotomy of the measurement, since half of the exposure time must be devoted to each measurement, to ⁇ and ⁇ + ⁇ .
• this method therefore offers a photometric advantage that is rooted in the fact that the two possible 45 degree projections are processed. All available energy is used. However, the quantity Im does not separate at 2 * from ⁇ , considering that I have known. There remains an ambiguity between the amplitude and the cosine of the phase.
• a measurement is carried out according to four phases. Before the Wollaston prism, 50% of the energy is removed via a semi-reflective plate. On the deviated beam, where we find the radiation of the two arms still polarized perpendicularly, we install a quarter wave plate whose axes are aligned with the two incident polarizations.
• a A 1/2 (a ⁇ + a 2 )
• a B 1/2 (a ⁇ -a 2 )
• Ac 1/2 (a ⁇ + a 2 [ ⁇ / 2])
• a D 1/2 (a ⁇ -a 2 [ ⁇ / 2]) where a 2 [ ⁇ / 2] represents the amplitude of the radiation at 2 shifted by ⁇ / 2.
• I A 1/4 (a ⁇ 2 + a 2 2 + 2a ⁇ .a 2 * .cos ( ⁇ ))
• I B 1/4 (a ⁇ 2 + a 2 2 -2a ⁇ .
• I c 1/4 (a ⁇ 2 + a 2 2 + 2a ⁇ .a 2 * .cos ( ⁇ + ⁇ / 2))
• I D 1/4 (a ⁇ 2 + a 2 -2a ⁇ .a 2 * .cos ( ⁇ + ⁇ / 2))
• I A 1/4 (a ⁇ 2 + a 2 2 + 2a ⁇ .a 2 * .cos ( ⁇ ))
• I B 1/4 (a ⁇ 2 + a 2 2 -2a ⁇ .a 2 * .cos ( ⁇ ))
• I c 1/4 (a ⁇ + a 2 2 + 2a ⁇ .a 2 * .cos ( ⁇ + ⁇ / 2))
• l D 1/4 (a ⁇ 2 + a 2 2 -2a ⁇ .a 2 * .cos ( ⁇ -l- ⁇ / 2))
• the system includes a Michelson-type interferometer, having a measuring arm for illuminating the eye and collecting the reflected light, and a reference arm for illuminating a moving mirror for deep exploration of the retinal tissue.
• the interferometer is used in rectilinear and perpendicularly polarized light in both arms.
• the light source S is a diode with a small temporal coherence length (for example, 12 ⁇ m) whose spectrum is centered on 780 nm. It gives in principle to the in vivo tomography system an axial resolution equal to half the coherence length divided by the refractive index of the medium. This light source S can be drawn. In this case, it is then synchronized with image capture and adaptive correction.
• the beam is limited by a field diaphragm corresponding to 1 degree in the field of view of the eye (300 ⁇ m on the retina) and a pupillary diaphragm corresponding to a 7 mm opening on an enlarged eye.
• An input polarizer P allows optimal balancing flows injected into the two arms of the interferometer.
• the two arms have a configuration called Gauss, afocal, which allows the transport of the pupils, on the one hand, and the materialization of an intermediate image of the field where a diaphragm blocks a large part of the corneal reflection, on the other hand.
• Quarter wave plates provide by rotation of the polarization of the single light reflected by the eye, and the moving mirror, effective filtering parasitized reflections in the in vivo tomography system according to the invention.
• the reference arm is similar to the measuring arm, but with static optics.
• the detection path of the in vivo tomography system according to the invention will now be described.
• the two beams on the output arm are still polarized perpendicularly, and they interfer only if they are projected on a common direction.
• a prism of Wollaston W has for function of simultaneously projecting the two radiations on two perpendicular directions of analysis.
• the detector is of the CCD type, with an image rate of more than 30 frames per second.
• This detector is associated with a dedicated computer (not shown) in which the digital image processing is performed: extraction of the four measurements, calibration, calculation of the amplitude of the fringes.
• the adaptive correction of the wave fronts is performed upstream of the interferometer, thus in the measuring arm. Each point of the source S thus sees its image on the corrected retina of the aberrations, and the image in return is also corrected. The amplitude of the fringes is then maximum.
• the adaptive optics subsystem includes a deformable mirror MD.
• the wavefront measurement is made by a Shack-Hartmann SH analyzer on the return beam of a light spot itself imaged on the retina via the deformable mirror MD.
• the wavelength of analysis is 820 nm.
• the illumination is continuous and provided by a superluminescent SLD diode temporally incoherent.
• the sizing of the analyzer corresponds to an optimization between photometric sensitivity and sampling of the wavefront.
• the rate of refreshing of the control of the deformable mirror MD can reach 150 Hz.
• a dedicated computer (not shown) manages the adaptive optics loop. The command however, is synchronized to freeze the shape of the mirror during the interferometric measurement.
• An appropriate control of the focusing of the analysis channel, by means of a lens LA2, makes it possible to adapt the focusing distance to the layer selected by the interferometer. This arrangement is essential to maintain optimal contrast at any depth.
• the deformable mirror MD is conjugated with the pupil of the system and the eye.
• the system field is defined by the system input DCM field iris. It is chosen equal to 1 degree, less than the isoplanetism field of the eye, which guarantees the validity of the adaptive correction in the field on the only wavefront measurement made from the spot, in the center of the field.
• the rotation of the deformable mirror MD makes it possible to choose the angle of arrival of the beam in the eye, and thus the portion of retina studied.
• the OCT measurement assumes the equality of the optical paths between the two arms of the Michelson interferometer, to the coherence length of the source. It also assumes an optimal focus on the depth that corresponds to this equality.
• the limitation of the beam diameter gives the eye a very large depth of field that dispenses with any re-focusing.
• the wavefront analyzer When the system is used at full aperture (typically F / 3), the depth of field decreases rapidly, typically 30 ⁇ m.
• the Z-scan of the OCT can rapidly exit this interval, beyond which the interferometric contrast decreases. This can be considered as an aberration effect of pure defocus.
• This problem can be remedied by providing the wavefront analyzer with a device for adjusting its own focus, for example with a mechanical adjustment. An arbitrary modification of this focusing forces, through the adaptive optics loop, the deformable mirror to adopt an additional curvature, combining input source and detector with a more or less deep point in the retina.
• the control of this focus must be synchronized with the Z-scan of the OCT. It is also possible to control the analyzer to force it to work defocused.
• An alternative solution to a real defocusing of the analyzer may be to add a pure focus term in the control of the mirror, whatever the measurement of the analyzer.
• This device is commonly used in adaptive optics.
• the so-called "reference slope" table is simply modified, which forces the system to converge on an arbitrarily modified command.
• An alternative solution to a real defocusing of the analyzer may be to add a pure focus term in the control of the mirror, whatever the measurement of the analyzer. This device is commonly used in adaptive optics.
• a transmission adaptive corrector system may be used in preference to fixed lenses for optimal correction.
• a collaborative or active sighting system is installed upstream of the assembly. This aiming system, which includes an active MAM pattern, presents to the subject the image of a light point that departs periodically from the desired line of sight. The patient is then invited to follow all the movements of this image.
• the in vivo tomography system is relatively compact, less than 1.2 m on one side.
• the diameter of the deformable mirror MD which partly fixes the focal length of the off-axis parabolas.
• the use of micro-mirrors obviously diminishes all the dimensions of the system.
• the detection system with its division into two beams, is realized here with discrete components. It is conceivable to make and use integrated components combining the functions of separation, aliasing, or even delay of the beams.
• the installation of the reference source SLD upstream of the deformable mirror MD allows an optimal quality of measurement of the aberrations and therefore of their compensation, since the reference image materialized in the eye benefits in this case from the adaptive correction.
• the reference source SLD is positioned closer to the eye in the optical path, in particular after the adaptive optics (on the go) and for example before a birefringence compensator, such as a solar compensator. Babinet CBC, or just before the eye. Failing to benefit from an optimal image spot in the back of the eye, the system then gains in operating stability.
• the deformable mirror used in the adaptive optics may be, for example, a 50 mm diameter mirror with 31 elements from CILAS.
• the performance and / or compactness of the device can be improved by using a more efficient model and / or more compact such as the deformable mirror of diameter 15 mm to 52 elements developed at the Astrophysics Laboratory of the Observatory of Grenoble, especially because of its compactness and a greater race in adaptation movements.
• a more efficient model and / or more compact such as the deformable mirror of diameter 15 mm to 52 elements developed at the Astrophysics Laboratory of the Observatory of Grenoble, especially because of its compactness and a greater race in adaptation movements.
• the folding mirrors of the measuring arm MPM1, MPM2 and those of the reference arm MPR1, MPR2 have been removed.
• the optical path of the measuring arm comprises a doublet of two lenses LM1-1 and LM2-2 on one side of the deformable mirror MD, and another pair of two lenses LM2-1 and LM2-2 on the other side thereof. deformable mirror.
• the optical path of the reference arm comprises a doublet of two lenses LR1-1 and LR2-2 on one side of the reference mirror MR, and another pair of two lenses LR2-1 and LR2-2 of the other side of this reference mirror.
• the use of lenses rather than mirrors may be more economical and allow better performance, in particular because of the cost and optical aberrations of this type of mirror, which are typically off-axis parabolic mirrors.
• the combination of such an assembly in the axis, with a smaller deformable mirror provides a more efficient system, simpler, or more economical, while maintaining a small footprint. As illustrated in FIG.
• the system may furthermore comprise conventional imaging means, such as an IMG camera, making it possible to associate the interferometric measurements with a simple imaging of the zones examined, for example to facilitate the exploration and selection of the areas to be examined.
• conventional imaging means such as an IMG camera
• a second polarizing cube CNPI places directly at the output (at the return) of the measuring arm, thus just before the CPR polarizer cube of the interferometer, a second polarizing cube CNPI makes it possible to divert the return beam towards an IMG imaging camera having its own means of LI focus of the image. On this path, a direct image of the targeted retinal area will be observable.
• the measuring arm and this additional channel can be arranged so that they provide a wider field of observation than the mode interferometric, whose field is limited in particular by the interferometric contrast measurement technique itself.
• the input source S Due to its short coherence length, the input source S has a polychromatic type spectrum. In a typical OCT assembly, this spectrum is generally relatively narrow, for example of a width of the order of 50 nanometers, but not necessarily negligible. This polychromatic spectrum can cause a degradation of the performances, in particular by causing a dispersion of the differences of step because of the dispersive nature of the ocular medium, which leads to a degradation of the axial resolution of the device. To avoid or limit these impairments, the system may include compensation means located in the reference arm.
• the system can then include compensation means located for example in the measuring arm.
• these means can compensate for a focal chromaticism which represents approximately 400 micrometers between the red and the blue, for example by replacing the LM2-2 collimator situated just in front of the eye by a doublet with a deliberately chosen chromatism opposite to that of the eye.
• These means can also compensate for differences in optical path due to chromatic dispersion, for example by inserting a water tank in the reference arm of a size dependent and / or adjustable depending on the size or characteristics of the eye to examine.
• Such a tank may be of a size of the order of 24 mm, the average length of a human eye.
• the use of these compensation means can improve the axial resolution by reducing it by a value of about 6 micrometers to a value about 4 micrometers.
• the system can also use as an input source of the interferometer a polychromatic illumination with a wider spectrum, for example in white light. In this case, the increase in performance provided by these compensation means will be much greater.
• the invention may in particular be implemented to make or complete a retinal imaging device, or corneal topography, or measuring a film of tears.
• the invention is not limited to the examples that have just been described and many adjustments can be made to these examples without departing from the scope of the invention.

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